Middle ear inflammations and resulting complications represent an extremely common health and social problem. The etiology and pathogensis of the various inflammatory middle ear diseases beginning with the middle ear effusions, such as serous otitus media and including the various forms of chronic otitus media are largely unknown. Through a study of these mechanisms, improved methods of treatment and prevention will result. Another important aspect is what appears to be a common complication of otitus media, sensorineural hearing loss. This phenomenon may be more prevalent in causing sensorineural hearing loss in adults than is presently realized. Our objective is to study the etio-pathogenesis of the middle ear inflammatory diseases which can occur along a continuum and may be dynamically interrelated, as well as the pathogenesis of sensorineural hearing loss as it may develop from round window diffusion of toxins. We concentrate our study on the precursor state (middle ear effusions) leading to chronic otitus media and study the immediate and advanced cellular changes of the inflammatory diseases. This multi-disiplinary study in both human and animal subjects is expanded to include methods applicable to the following disciplines: 10 morphology and pathology (normal base line information and induced cellular changes including histochemistry, autoradiography and electronmicroscopy); 2) biochemistry (chemical alterations of the middle ear effusions and perilymph in serous labrynthitis); 3) functional assessment (human and animals) to investigate very high tone losses. Bibliographic references: Definitions and Terminology. Michael M. Paparella, M.D. Presented at the International Symposium on Recent Advances in Middle Ear Effusions, May 29, 1975. To be published in the Annals. Summary. Michael M. Paprella, M.D. Presented at the International Symposium on Recent Advances in Middle Ear Effusions, May 31, 1975. To be published in Annals.